The present invention relates to a method in a digital radio system for signalling between a base station and a subscriber terminal, in which the signals transmitted over the radio connection between the base station and the subscriber terminal comprise bursts consisting of symbols, said bursts comprising a known training sequence.
In a radio system, two kinds of signallings are employed: signalling that is related to a specific call, and signalling that takes place independent of the call.
Call-related signalling takes place at the same time as the user""s data flow is being transferred. Prior art in such a case knows two options for the transmission path: SACCH and FACCH. SACCH is a low-rate, separate channel associated with each traffic channel. Due to its low rate, it may only be used for non-urgent signalling, e.g. for transmitting measurement results concerning the radio path. FACCH is signalling within the traffic channel. Due to its high rate, it may be employed for a more time-critical signalling, e.g. for authenticating a subscriber, or for signalling associated with a handover. A major drawback is that when occurring during a call, the FACCH has priority over the user""s data flow, whereby user information will be lost, which results in a similar outcome as errors in the transmission.
Call-independent signalling takes place when there is need to set up a connection between a base station and a subscriber terminal only for signalling purposes, e.g. for conveying short messages. In prior art solutions, traffic channels may be used for this, either at half or full rate. However, this means that resources are wasted, as the channels are in such a case normally used for transmitting but a few messages and only for a very short time. As a consequence, SDCCH has been developed, having the rate as low as one eighth of that of a normal traffic channel. This channel may be used also for e.g. a slow data transmission service, but that leads to the problem met in the FACCH signalling: user data flow may be lost due to signalling having a priority over it.
The following discusses transferring a transmitting power control command from a base station to a subscriber terminal. The simplest way of implementation is to define a power control bit. Depending on the value of the bit, the transmitting power is either reduced or increased. Additionally are needed several error correction bits, which are used to ensure that also during a bad radio connection the correct command can be conveyed. This is most important, because during a bad radio connection one way of remedy is to increase the transmitting power. In practice, if this is carried out by e.g. FACCH signalling, at least three bits per burst are needed for the transmission of the transmitting power control command and the associated error correction bits. This results in a traffic requirement of at least 650 bit/s, which is off the capacity that would be needed on the traffic channel for transferring the user""s data flow.
A training sequence refers to a group of predetermined symbols known by the receiver. By comparing the received training sequence by means of specific methods to the known training sequence, the receiver is able to deduct what kind of distortions, caused by a non-ideal radio connection between the base station and the subscriber terminal, are present in the received signal, and by utilizing this information the receiver may more easily demodulate the received signal.
The severest drawbacks of the prior art thus have to do with the low rate of signalling and disturbance it causes to the transfer of user data flow, or the obstacles it creates to developing new services in the future.
It is consequently the object of the present invention to implement a signalling method by means of which the aforementioned drawbacks may be avoided.
This is achieved with a method of the type set forth in the introduction, characterized in that a training sequence is used for signalling.
The invention further relates to a digital radio system for signalling between a base station and a subscriber terminal, in which the signals transmitted over the radio connection between the base station and the subscriber terminal comprise bursts consisting of symbols, said bursts comprising a known training sequence, the system having at least one transmitter and at least one receiver, of which the transmitter has means for inserting the training sequence in the signal to be transmitted, and of which the receiver has means that may be employed for identifying the training sequence in the received signal, characterized in that the transmitter comprises means which, when used for data transmission, convert data to be presented as a predetermined training sequence, and the receiver comprises means for converting the training sequence identified in the received signal into the data that was intended to be transmitted to it.
The method of the invention provides a plurality of advantages. A most significant advantage is obtained by the fact that the rate of the traffic channel will not decrease; its payload may thus be used entirely for transferring the user data flow. Consequently, as regards the user, there is no interference caused by the traffic channel being used for signalling.
Another most useful characteristic stems from the reliability of the signalling. As the signalling information is coded into a training sequence, its error-free transfer from a base station to a subscriber terminal is most reliable as a result of the advanced methods used for identifying the training sequence.
It is a further advantage of the solution that it is possible in the future to develop new services that employ low-rate transmission channels, and as signalling is not carried out on these channels, no disturbance will be generated, either.